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>13.1. Introduction</A
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><P
>    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> provides a rich set of tools
    for developers to manage concurrent access to data.  Internally,
    data consistency is maintained by using a multiversion
    model (Multiversion Concurrency Control, <ACRONYM
CLASS="ACRONYM"
>MVCC</ACRONYM
>).
    This means that while querying a database each transaction sees
    a snapshot of data (a <I
CLASS="FIRSTTERM"
>database version</I
>)
    as it was some
    time ago, regardless of the current state of the underlying data.
    This protects the transaction from viewing inconsistent data that
    could be caused by (other) concurrent transaction updates on the same
    data rows, providing <I
CLASS="FIRSTTERM"
>transaction isolation</I
>
    for each database session.  <ACRONYM
CLASS="ACRONYM"
>MVCC</ACRONYM
>, by eschewing
    the locking methodologies of traditional database systems,
    minimizes lock contention in order to allow for reasonable
    performance in multiuser environments.
   </P
><P
>    The main advantage of using the <ACRONYM
CLASS="ACRONYM"
>MVCC</ACRONYM
> model of
    concurrency control rather than locking is that in
    <ACRONYM
CLASS="ACRONYM"
>MVCC</ACRONYM
> locks acquired for querying (reading) data
    do not conflict with locks acquired for writing data, and so
    reading never blocks writing and writing never blocks reading.
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> maintains this guarantee
    even when providing the strictest level of transaction
    isolation through the use of an innovative <I
CLASS="FIRSTTERM"
>Serializable
    Snapshot Isolation</I
> (<ACRONYM
CLASS="ACRONYM"
>SSI</ACRONYM
>) level.
   </P
><P
>    Table- and row-level locking facilities are also available in
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> for applications which don't
    generally need full transaction isolation and prefer to explicitly
    manage particular points of conflict.  However, proper
    use of <ACRONYM
CLASS="ACRONYM"
>MVCC</ACRONYM
> will generally provide better
    performance than locks.  In addition, application-defined advisory
    locks provide a mechanism for acquiring locks that are not tied
    to a single transaction.
   </P
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